Before gene therapy can become clinically practical, methods will need to be developed for accurately assaying the in vivo expression of genes that have been delivered to a patient. Ideally, such methods should reveal not only where within the patient's body expression is occurring, but also whether it is taking place at a level and for a duration sufficient to be therapeutically effective. Most of the methods that have been developed thus far rely upon gamma-camera, SPECT or PET imaging to detect injected radiolabeled compounds. For example, Herpes simplex virus 1 thymidine kinase gene transfer has been followed using γ-camera imaging and positron emission tomography (PET) of radiolabeled prodrugs (Tjuvajek, et al., Cancer Res. 58:4333–4341 (1998); Alauddin, et al., Nucl. Med. Biol. 25:175–180 (1998); Gambhir, et al., Med. Sci. 96:2333–2338 (1999)). Transfer of the type 2 dopamine receptor has been detected by PET using labeled antagonists (Gambhir, et al., Nucl. Med. Biol. 26:481–490 (1999); MacLaren, et al., Gene Ther. 6:785–791 (1999)) and transfer of a rat sodium/iodide symporter has been followed using a γ-camera to detect intracellularly trapped radioactive iodine (Mandell, et al., Cancer Res. 59:661–668 (1999)). One problem with many of these methods is that they either employ radiopharmaceuticals that are not known to be safe for use in humans or they use radioisotopes in ways that may have unforeseen adverse consequences.
Somatostatin receptors belong to a class of G-protein associated receptors having similar predicted three-dimensional structures consisting of seven transmembrane domains bridged by extracellular and intracellular loops. The somatostatin receptor family includes at least six distinct receptor subtypes encoded by five different genes, one of which generates two splice variant mRNAs. Gene sequences encoding human, rat, and, in some cases mouse somatostatin receptor (SSTR) subtypes 1, 2, 2b, 3, 4 and 5 have been published in the literature (Bruns et al., Ann. NY Acad. Sci., 733:138–146, 1994 and references cited therein). Accession numbers for exemplary mRNAs encoding these receptors can be found below.
The somatostatin type 2 receptor is characterized by the presence of an extracellular domain, seven transmembrane domains, and an intracellular domain that appears to be responsible for receptor internalization. The type 2 receptor is divided into two different subforms, 2 and 2b, that are identical except that type 2 has a longer C-terminal cytoplasmic (i.e. intracellular) domain. In vivo, the type 2 receptor has been detected using 111In-labeled octreotide (John, et al., Gut 38:33–39 (1995)), and somatostatin analogues labeled with either 99mTc or 188Re (Zinn, et al., J P Nucl. Med. 41:887–895 (2000)). Currently, the 188Re analog is not approved for therapeutic use by the FDA and the 99mTc analog is only approved for imaging lungs. In contrast, 111In octreotide has been approved for total body imaging. Systems for following gene transfer which rely upon the imaging of a somatostatin receptor would provide a new tool for evaluating and monitoring gene therapy.